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          <h1 class="post-title" itemprop="name headline">C/C++内存对齐策略</h1>
        

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        <p>struct内存补齐属于笔试中的必考题目之一，但有时候stuct内存对齐的方式对让人感到非常疑惑，这里梳理一下内存对齐的策略。</p>
<a id="more"></a>
<hr>
<h1 id="策略"><a href="#策略" class="headerlink" title="策略"></a>策略</h1><p>规则一：前面的地址必须是后面地址的整数倍，不是就对齐；</p>
<p>规则二：整个地址必须是最大字节的整数倍；</p>
<p>规则三：指定#pragma pack(n)，整个地址是n的整数倍。</p>
<h1 id="实例"><a href="#实例" class="headerlink" title="实例"></a>实例</h1><p>以32位环境为例（64位环境指针为8个字节，规则相同）：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br><span class="line">93</span><br><span class="line">94</span><br><span class="line">95</span><br><span class="line">96</span><br><span class="line">97</span><br><span class="line">98</span><br><span class="line">99</span><br><span class="line">100</span><br><span class="line">101</span><br><span class="line">102</span><br><span class="line">103</span><br><span class="line">104</span><br><span class="line">105</span><br><span class="line">106</span><br><span class="line">107</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">s</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">    <span class="keyword">char</span> *p; <span class="comment">// 4字节</span></span><br><span class="line">    <span class="keyword">char</span> c;  <span class="comment">// 4字节 规则一</span></span><br><span class="line">    <span class="keyword">int</span> a;   <span class="comment">// 4字节</span></span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">12</span>;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">s</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">    <span class="keyword">char</span> *p;    <span class="comment">// 4字节</span></span><br><span class="line">    <span class="keyword">char</span> c;     <span class="comment">// 1字节</span></span><br><span class="line">    <span class="keyword">char</span> ch[<span class="number">2</span>]; <span class="comment">// 3字节 规则一</span></span><br><span class="line">    <span class="keyword">int</span> x;      <span class="comment">// 4字节</span></span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">12</span>;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">s</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">    <span class="keyword">char</span> p; <span class="comment">// 4字节 规则一</span></span><br><span class="line">    <span class="keyword">int</span> x;  <span class="comment">// 4字节</span></span><br><span class="line">    <span class="keyword">char</span> c; <span class="comment">// 4字节 规则二</span></span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">12</span>;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">s</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">    <span class="keyword">char</span> p; <span class="comment">// 4字节 规则一</span></span><br><span class="line">    <span class="keyword">int</span> x;  <span class="comment">// 4字节</span></span><br><span class="line">    <span class="keyword">char</span> c; <span class="comment">// 4字节</span></span><br><span class="line">    <span class="class"><span class="keyword">struct</span> <span class="title">a</span></span></span><br><span class="line"><span class="class">    &#123;</span></span><br><span class="line">        <span class="keyword">char</span> *b; <span class="comment">// 4字节</span></span><br><span class="line">        <span class="keyword">long</span> y;  <span class="comment">// 4字节</span></span><br><span class="line">    &#125; inner;     <span class="comment">// 8字节</span></span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">20</span>;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">s</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">    <span class="keyword">char</span> p;</span><br><span class="line">    <span class="keyword">char</span> x;</span><br><span class="line">    <span class="class"><span class="keyword">struct</span> <span class="title">a</span></span></span><br><span class="line"><span class="class">    &#123;</span></span><br><span class="line">        <span class="keyword">char</span> b;</span><br><span class="line">        <span class="keyword">int</span> y[<span class="number">10</span>];</span><br><span class="line">    &#125; inner;</span><br><span class="line">    <span class="keyword">char</span> c;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">56</span>;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">s</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">    <span class="keyword">char</span> p;</span><br><span class="line">    <span class="keyword">char</span> x;</span><br><span class="line">    <span class="keyword">int</span> f : <span class="number">1</span>;</span><br><span class="line">    <span class="keyword">int</span> b : <span class="number">2</span>;</span><br><span class="line">    <span class="keyword">char</span> c : <span class="number">1</span>;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">12</span>;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">s</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">    <span class="keyword">char</span> c; <span class="comment">// 4字节 规则一</span></span><br><span class="line">    <span class="keyword">union</span> u &#123;</span><br><span class="line">        <span class="keyword">char</span> *p;</span><br><span class="line">        <span class="keyword">short</span> x;</span><br><span class="line">    &#125; inner; <span class="comment">// 四字节</span></span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">8</span>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">union</span> s &#123;</span><br><span class="line">    <span class="keyword">char</span> c;</span><br><span class="line">    <span class="class"><span class="keyword">struct</span> <span class="title">u</span></span></span><br><span class="line"><span class="class">    &#123;</span></span><br><span class="line">        <span class="keyword">char</span> p;</span><br><span class="line">        <span class="keyword">int</span> x;</span><br><span class="line">    &#125; inner;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">8</span>;</span><br><span class="line"></span><br><span class="line"><span class="meta">#<span class="meta-keyword">pragma</span> pack(2)</span></span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">s</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">    <span class="keyword">char</span> *p; <span class="comment">// 4字节</span></span><br><span class="line">    <span class="keyword">char</span> x;  <span class="comment">// 2字节 规则三</span></span><br><span class="line">    <span class="keyword">int</span> a;   <span class="comment">// 4字节</span></span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">10</span>;</span><br><span class="line"></span><br><span class="line"><span class="meta">#<span class="meta-keyword">pragma</span> pack(1)</span></span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">s</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">    <span class="keyword">char</span> *p;</span><br><span class="line">    <span class="keyword">char</span> x;</span><br><span class="line">    <span class="keyword">int</span> a;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">9</span>;</span><br><span class="line"></span><br><span class="line"><span class="meta">#<span class="meta-keyword">pragma</span> pack(8)</span></span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">s</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">    <span class="keyword">char</span> *p;</span><br><span class="line">    <span class="keyword">char</span> x;</span><br><span class="line">    <span class="keyword">int</span> a;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">sizeof</span>(s) = <span class="number">16</span>;</span><br></pre></td></tr></table></figure>

      
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